Gold

Class: Non-Ferrous Metals

Group: Noble metals

Metal: Gold

Chemical formula: Au

Ingredients: 57-99 % Au

Mohs scale hardness: 2,5-3

Specific gravity: 15,6-19,2 g/cm³

Magnetic properties: Nonmagnetic

During the processing of minerals containing gold, plants use the following types of separation:

• Gravity separation

• Flotation separation

• Chemical separation

• Special methods of separation and Magnetic separation (the separation of gold from chips)

• Amalgamation

Promising methods for the extraction of golden ore:

• Magnetic separation

• Electrical separation

• Special methods of separation

• Segregation-diffusion concentration

Following methods of ore conditioning are used in processing of golden ore:

• Oxidizing roasting

• Bacterial oxidation (Acidithiobacillus ferrooxidans, Sulfobacillus)

• Autoclave oxidation

There are three main sources of mineral raw materials for the extraction of gold:

1. Placer deposits of ores containing gold (alluvial deposits)

2. Indigenous ore vein deposits containing gold

3. Waste and not the traditional raw materials containing gold

In nature, there are more than 20 mineral forms of gold. It is difficult to choose a method for separating of gold. Minerallurgy can significantly reduce the cost of reagents for hydrometallurgy and reduce the volume of leached ore. For some types of ores the minerallurgy is the sole method of separation of gold in front of pyrometallurgy.Traditional methods of extraction of gold:

Gravity separation. Gravity separation is used to allocate fairly large particles of gold of placer deposits. Gravity separation is used prior to flotation to separate golden particles greater than 0.1 (0.15) mm too. The lower limit size of the use of gravity separation to date is 5 microns. Major losses in golden gravity separation comprise splices of gold with other minerals, platelet-shaped particles of gold, micro and nanoparticles of gold. Recovery of gold of size of 50-100 microns is 96 %.

Flotation separation. Flotation is the most common and effective way of golden separation, because you can extract the free gold and splices of gold. Floatability of gold depends on the nature of gold, size of fine particles of gold, shape of gold particles, chemical purity of gold, presence of impurities, composition of the films on the surface of golden particles, material composition of the ore. When froth flotation can be extracted particles with a size 150-10 microns. For golden particles less than 20 microns is best to use special methods of flotation. In addition, the golden particles of different size of one field have different flotation properties.

Chemical enrichment (chemical separation, hydrometallurgy). Heap leaching of ore or concentrate is used mainly with cyanide (recovery of gold is around 60-90 %). Situ leaching is used with chlorine too (extraction of gold may reach 75 %). Also there are pressure leaching and bacterial leaching. Chemical processing is not used for macrogold and colloidal gold. Leaching uses the following types of solvents: cyanides, hypochlorite and other solutions (thiourea). To separate the gold from solution can be used metal deposition, electrolysis, adsorption with activated carbon, sorption of synthetic ion-exchange resins.

Composition of the golden ore often contains the following minerals: quartz, pyrite, arsenopyrite, gray ores, sphalerite, galena, chalcopyrite, magnetite, barite, tourmaline, carbonates, bismuthine.

Composition of natural gold includes the following admixtures: silver, copper, lead, bismuth, antimony, iron, tellurium, and arsenic. Impurity complicates the selection process of method for gold separation and development of technology to extract gold.

Impurity reduces the value of gold and changes its properties:

• Impurity reduces the density of gold.

• Impurity alters the structure of gold.

• Impurity alters the color of gold (from pale yellow to reddish or greenish).

• Impurity of iron gives the magnetic properties of gold.

Gold is classified by the following features:

By mineral form:

• Native gold (it can be extracted by gravity separation, chemical separation)

• Sulphides (gold can be extracted by flotation separation, chemical separation)

• Galloidy (gold can be extracted by chemical separation)

• Carbonaceous ore and other forms.

By connection with other minerals:

• Free gold (it can be extracted by gravity separation, flotation, chemical separation)

• Splice of gold (gold can be extracted by flotation, chemical separation)

• Gold that enclosed in a mineral or inside the crystal lattice (gold can be extracted by flotation, pyrometallurgy)

By size of particles:

• Macrogold is very large (greater than 0.5 mm). It can be extracted by gravity concentration.

• Coarse gold (fat gold) is in a free state (70 micron). It is easily releases during grinding. It can be extracted by gravity concentration. Coarse gold floats badly. For large-scale leaching of gold requires a lot of time.

• Fine gold is partly in a free state and partly in the splice (70-1 micron). Free fine gold floats well and quickly dissolves in the leaching. Using of gravity separation is possible, but it difficult. Fine gold with splice recoveries by flotation too. Fine gold in intergrowths with sulfides floats well, fine golden splice with nesulfidami floats much worse. Fine golden splice is well dissolved in the leaching.

• Microgold is finely divided (less than 1 micron). When grinding ore microgold opens slightly, the bulk of gold remains in the mineral. Most often, these minerals are pyrite and arsenopyrite. The content of gold in pyrite may be several grams per ton. For the separation of golden splices used flotation and chemical separation. Splices of gold with iron hydroxides, carbonates are leached well even with rough grinding. Splices of gold with sulphides after flotation is subjected to oxidizing roasting of sulfides to decompose, and then are leached (or immediately directed to the pyrometallurgy). Splices of gold with quartz after flotation are sent to pyrometallurgy.

• Nanogold is disseminated nanoparticles of gold on minerals. Nanogold as fine disseminations may be present in the following sulfides: pyrite, arsenopyrite, pyrrhotite, chalcopyrite, galena, sphalerite, covellite, bismuthine. Nanogold presents in the auriferous quartz in the form of nano disseminations. Also nanogold cans be inside the crystal lattices of other minerals. Only special methods can be used to extract nanogold.

• Colloidal gold is an unstable form of gold. The size of colloidal gold particles is not strictly set. The developed technology of separation of colloidal gold doesn’t exist now.